17.26 Emulating TLS

For targets whose psABI does not provide Thread Local Storage via
specific relocations and instruction sequences, an emulation layer is
used. A set of target hooks allows this emulation layer to be
configured for the requirements of a particular target. For instance
the psABI may in fact specify TLS support in terms of an emulation
layer.

The emulation layer works by creating a control object for every TLS
object. To access the TLS object, a lookup function is provided
which, when given the address of the control object, will return the
address of the current thread's instance of the TLS object.

— Target Hook: const char * TARGET_EMUTLS_GET_ADDRESS

Contains the name of the helper function that uses a TLS control
object to locate a TLS instance. The default causes libgcc's
emulated TLS helper function to be used.

— Target Hook: const char * TARGET_EMUTLS_REGISTER_COMMON

Contains the name of the helper function that should be used at
program startup to register TLS objects that are implicitly
initialized to zero. If this is NULL, all TLS objects will
have explicit initializers. The default causes libgcc's emulated TLS
registration function to be used.

— Target Hook: const char * TARGET_EMUTLS_VAR_SECTION

Contains the name of the section in which TLS control variables should
be placed. The default of NULL allows these to be placed in
any section.

— Target Hook: const char * TARGET_EMUTLS_TMPL_SECTION

Contains the name of the section in which TLS initializers should be
placed. The default of NULL allows these to be placed in any
section.

— Target Hook: const char * TARGET_EMUTLS_VAR_PREFIX

Contains the prefix to be prepended to TLS control variable names.
The default of NULL uses a target-specific prefix.

— Target Hook: const char * TARGET_EMUTLS_TMPL_PREFIX

Contains the prefix to be prepended to TLS initializer objects. The
default of NULL uses a target-specific prefix.

— Target Hook: tree TARGET_EMUTLS_VAR_FIELDS (tree type, tree *name)

Specifies a function that generates the FIELD_DECLs for a TLS control
object type. type is the RECORD_TYPE the fields are for and
name should be filled with the structure tag, if the default of
__emutls_object is unsuitable. The default creates a type suitable
for libgcc's emulated TLS function.

Specifies a function that generates the CONSTRUCTOR to initialize a
TLS control object. var is the TLS control object, decl
is the TLS object and tmpl_addr is the address of the
initializer. The default initializes libgcc's emulated TLS control object.

— Target Hook: bool TARGET_EMUTLS_VAR_ALIGN_FIXED

Specifies whether the alignment of TLS control variable objects is
fixed and should not be increased as some backends may do to optimize
single objects. The default is false.

— Target Hook: bool TARGET_EMUTLS_DEBUG_FORM_TLS_ADDRESS

Specifies whether a DWARF DW_OP_form_tls_address location descriptor
may be used to describe emulated TLS control objects.